Electrical amplifier circuit



Nov. 24, 1931.

F. H. DRAKE'ET AL ELECTRICAL AMPLIFIER CIRCUIT Filed June 7, 1928 4 Sheets-Sheet 1 Nov. 24, 1931; DRAKE ET AL 1,833,638

ELECTRICAL AMPLIFIER CIRCUIT Filed June '7, 1928 4 Sheets-Sheet 2 Nov.

24, 1931. F. H. DRAKE ETAI.

ELECTRICAL AMPLIFIER CIRCUIT Filed June 7, 1928 4 Sheets-Sheet 3 M'cra vii/bra fired: 5 4069 Came/'7} Nov. 24, 1931. DRAKE T L 1,833,638

ELECTRICAL AMPLIFIER CIRCUIT Filed June '7, 1928 4 Sheets-Sheet 4 WOW attoznet s Patented Nov. 24, 1931 UNITED STATES PATENT OFFICE FREDE ICK H.DRAK13 Jinn IfAlJ'L o, RARNHAivi, or noon'rou, new JERSEY, ASSIGNORS,

BY MESNE ASSIGNMENTS, roRAnIo CORPORATION or AMERICA, 0]? NEW Y0RK,,

N. Y., A CORPORATION OF DELAWARE ELEC'iRIcAL AMPLIFIER CIRCUIT A piiation fii'a June 7, 1928. serial No. 283,624.

j' rodes, and so forth. The invention is an improvement upon the invention described and claimed in United States Patent No. 1,672,811 to Lewis M. Hull, issued June 5, 1928, and on the invention describedand claimed in copending application of Frederic}; Drake, :Sei'ialNo, 275,178, filed May 4,1928, patented June 17, 1930, No. 1,764,552; and relates more particularly to an electrical amplifier circuit including means adapted to reduce or sup- {5 press certain effects other'than desired forwere repeater action of the tube, and adapted especially to reduce or suppress e'fi'ects due to the flow of retroactive or regenerative currents arising out of capacitive coupling between the outputandinput circuits ass'o'ciated with such a tube, this capacitive coupling including particularly capacity between the anode and control element of the tube, and capacity diie to connections and i0 circuits associated with said anode and coneleinent. f referring now to the drawings, Figure 1 represents one specific embodiment of the intent-ion, showing its application to a single 5 stage radio frequency electrical amplifier including a three-electrode tube. Figure 1a seats at anti-nativearrangement to that of Figure 1. Figure 1]) shows arrangement similar to that of Figured, b'ut in'cluding a "four-element er tetrode tube of the doubleiirid type: Figure 2 shows one specific enibtmliinent of the invention as applied to a 7 radio receiving set including a multi-stage radio frequency amplifier. Figures 8, 4 and 5 "are gra'phie representations showing certain reflects upon selectivity, balance tolerance, and

gain characteristic, respectively, arising out of the use of the present invention in certain specific instances.

Referring now to the particular circuit ar- 5 rangement shown in Figure 1, T is a tube of the three-electrode type, including an anode P, control element G, and cathode F, the latter being, in this particular instance, an alternating current filament heated by current from any suitable source, such as 110 volt 60 cycle house-lighting source, as indicated. In this case the step-down transformer Tr is emvto any desired value with respect tothe cathode by means of the resistance R0.

L -L constitutes an input transformer off WhICh the primary L may be associated with any desired source of electrical currents to be amplified, a connection to the plate circuit of a preceding tube being indicated, by way of example, by the inclusion between input:

terminal 2 and coilL of an equivalent plateresistance Rp equal to, say, 10,000 ohms, and by the connection to ground of the input terininal 1. The secondary L is tuned by variable tuning condenser C the voltage across which, represented by E0, constitutes the input voltage of the amplifier stage, being applied to the stage between the grid G and ground and measured when the tube is unlighted. The output circuit includes a transformer Il -L provided with primary winding L connected(t0gether with a suitable source of potential such as the B-battery B shunted by by-pass condenser C) between cathode and anode of the tube, and a secondary winding]; coupledthereto. L and L are so connected and wound as to have efi'ectively negative mutual inductance M between them, that is, L and L are wound 1n the same sense, connections to L being reversed, or alterna- Each side of the filament 1S360 tively, L and L are wound in opposite sense, connections being normal. A tertiary coil L which may be an element of the output transformer, as described and claimed in copending application of Frederick H. Drake and Villiam D, Loughlin, Ser. No. 275,449, filed May 5, 1928, is included in the tuned secondary circuit L -L C and preferably has negligible or substantially zero electromagnetic coupling with the coils L and L for example being arranged with its axis at right angles to the axes of L and L The junction ofthe coil L and the secondary tuning condenser C is grounded, as shown, and is thus connected to the cathode F which is by-passed to ground. The output voltage of the amplifier stage is the voltage E across the output terminals 8 and 4E. The ratio E/Eo therefore represents the gain or voltage amplification of the amplifier stage, and a curve obtained by plotting E /E0 against wave-length (A) is the gain or amplification curve of the amplifier.

In such an electrical circuit including an inductive, tuned, plate circuit, the capacitive coupling, C between anode P and control element G, together with the associated stray capacities, produces feed-back or retroactive currents which tend to causeregeneration, instability, and oscillation, as is well understood. This efiect is, of course, greater at short wavelengths than at long wavelengths since the impedance of C decreases with increasing frequency. In certain types of amplifier circuits, also, for instance circuits involving fixed inductances tuned to resonance at diiferent wavelengths by variable condensers, the amplification of the stage tends to vary with wavelength, assuming large val-V ues at the shorter wavelengths and smaller values at the longer wavelengths, all within the rang'e'of a given variable tuning condenser in combination with a given fixed coil. From the standpoint of stability and an economical design it is in general preferableto have substantially uniform amplification over the wavelength range used. In order to obtain this, it is, therefore, advantageous not only to suppress retroactive effects which would normally increase the overall amplification by regeneration, and thus produce instability, but, in addition, to further modify the behavior of the'stage in such a way that the net amplication at the shorter wavelengths is as low as, or even less than, the theoretical or so-called repeater amplification of a tube and transformer.

In order to balance the amplifier circuit as regards the capacitive currents fed back through the capacity G due to the current flowing through the tuned output circuit L L C it has been proposed to employ a balancing circuit arranged between the grid Q= e 0 L3.

wherein 1 C represents inter-electrode capacity between anode and control element of the tube, plus stray capacities of-leads,connections, sockets, and so forth.

C represents the balancing capacity M represents the mutual inductance between coils L and L L represents the self-inductance of the coil L 7 This equation, when approximately fulfilled, results in a substantial suppression of all first-order regenerative effects in the stage.

It has further been proposed (as described and claimed in the above-mentioned copending application of Frederick H. Drake, S81. No. 275,178, filed May 4, 1928) toproduce a controlled resistance unbalance in the presence of a substantial reactance balance, in such an amplifier network, by the inclusion in the primary or anode circuit of suitable resistance R This resistance R, maybe of any value in excess of the normal self-resistance of the primary circuit when L is wound with low-resistance wire, but is preferably of such value as to produce a flattening or increased uniformity inthe gain or amplification curve of the amplifier, as described above, by reducing the amplification at short wavelengths without substantially afi'ecting the amplification of longer wave-lengths. In Figure 1 the resistance R, is shown as com prised by the distributed self-resistance of the primary coil L this coil being wound, for example, of suitable resistance wire. R may, however, alternatively be comprised wholly or in part by a separate lumped resistance includedin circuit with the primary coil L The inclusion of resistance in the primary or anode circuit, as described, whether distributed or lumped, results in certain marked advantages over the arrange .ment employing merely a reactance balance as regards the secondary current, these advantages being explained in the above-mentioned copending application.

We have now found that a number of very considerable additional advantages may be obtained by the use of resistance R, of suitable value, suitably arranged, in the balancing circuit R O as shown. According to our invention, this resistance in the balancing circuit may be of any desired value in excess of the normal self-resistance of this circuit including the physical Condenser C but in a preferred arrangement, the amount of this resistance, R maybe so adjusted as to produce, to a-greater or less degree, the results shown in Figures 3, land 5, retaining at the same timethe advantages arising from the use of R and. is accordingly of such value as Y to partially or wholly overcome the resistance unbalance of the network produced by the resistance R in the primary circuit, thereby producing (to any desire degree) a resistance balance of the network not only in the presence of a substantial reactance balance thereof, but alsoin the presence of the added resistance R The advantages arising from the use of resistance R, in the primary circuitin excess of the normal resistance thereof, as described and claimed in the above-mentioned copending ap lication of Frederick H. Drake, Ser. No. 2 5,178, filed May 4,1928, are thus retained. according to the present invention, but certain marked additionaladvantages are also obtained, as will be hereafter more particularly described in connection with Figures 3, l and 5.

vAs a specific example, only, and without in any way limiting the invention thereto, wemay give the following constants for the resistance-capacity balance network of the radio frequency amplifier shown in Figure 1.

-Tabe=UX 22 6 (8.4 ,a. 7". average gridplate capacity) R0 set to ive rid bias =6 volts=E E 94, volts 7 Ef 1.5 Volts R' so ohms Gf=.5 a. f.

- Figure 1a represents an alternative arrangement to that of Figure 1,11 being constituted a separate lumped resistance in series between a low-resistance coil, L and theanode, rather than by the distributed selfresistance of the primary as in Figure 1, and

R being arranged in p arallel with the balancing capacity G Either of these variations may, of course, be employed without the other. Although the mathematical co'nditiQn for idealresistanc'e balance of the networkitheoretical'ly re uires that R, be in shunt with C we find that, in general, the

the terminals marked B and B 4 balance obtained in practice by the use of the resistance R, in series with C is preferable. The inherent resistance of C may, of course, in either the series or shunt arrangements, be made to include a part or all of the necessary resistance, R

Figure 1b shows one of the several arrangeinents according to which the circuit of Figure 1 may be applied to a double-grid tube. The inner grid G is here used as the control element while the outer grid G has applied to it a positive bias in order to reduce the capacity between anode and control grid. Since this capacity is not always thus wholly eliminated, for example where the outer grid G constitutes a volume control electrode as described and claimed in copending application of Frederick H. Drake, Ser. No. 281,936, filed May 81, 192, the use of a balanced circuit according to the present invention is found desirable. An alternative arrangement is to employ the outer grid G as the control element and apply a positive bias to the grid G The present invention may be applied to this arrangement as well. Other arrangements involving tetrodes, to which the present invention is applicable, are described in the above-mentioned United States Patent No. 1,672,811, to Lewis M. Hull, and application of Frederick H. Drake, Ser. No. 275,178, filed May 4, 1928.

Figure 2 represents the circuit of Figure 1 as applied, in one suitable manner, to the radio frequency amplifier of a radio receiving set. The tubes T T and T may suitably be UX-226 tubes, and T, a UX227 tube, for example; in which case the constants of the several circuits may suitably be the same as before. The input system may be of any desired form, and is here shown,- by way of illustration only, as that described and claimed in "cop'ending application of Frederick H. Drake and \Villiam D. Loughlin, Ser. No. 285,886, filed June 16, 1928. In this case the constants of the input system may suitably be as follows:

L0=24O It. max. 170' a. h. min.

R=5000O ohms volume control The condensers C are ganged and operated from a single control,- the condenser C in the input system being aligned with the others for any given antenna constants by means of the trinnning inductance L0. The output from the detector tube T 4 may suitably go to an audio frequency amplifier and loud speaker, as is well understood. The grids are biased by means of the resistance R0; a suitable source of alternating current is applied to the terminals marked A. C. Filament, and a suitable source of potential is applied between Bypass condensers C may be used in any required manner, for example as shown.-

The specific circuits and constants herein shown and described are examples, only, of circuits embodying the present invention, which is in no way limited thereto. We have found that in certain cases, for example, it is desirable to employ UX-227 or other tubes, instead oi UX-226 tubes, since the former tubes have lower grid-plate capacity. Tubes of the direct current filament type, obviously, may also be employed. Either socket-power devices or batteries may be used to supply the necessary potentials, and the grids may be biased in other ways, or left unbiased if desired. Othervariations in the circuit arrangements may, of course, be made equally with those mentioned. Obviously R R and G may be either fixed or adjustable in each case.

Certain of the advantages arising from the use of the present invention are illustrated Figures 3, sand 5. Figure 3 shows two selectivity curves obtained by plotting the gain (in percent of gain at resonant trequency) against the number of kilo-cycles oil resonance, for the circuit tuned to resonance at )r=225 meters wavelength. The curves were talren with a circuit having the constants given above in connection with Figure 1, (is being set by forward aural balance. It Will.

be noted that R =7O0 ohms, for the other particular circuit constants in this example," gives markedly increased selectivity over R =O- (i. e. when the present invention is not employed). lVith this value of R 700 ohms there is the further important advantage that the residual signal as C, is adjusted for balance is materially less than the lowest residual signal obtainable when R =0.

Figure at shows two curves representing the balance tolerance, with and without the use of the present invention, for the particular case of the circuit constants referred to. The curves are obtained by plotting Eo/Eo (where E6 is the 1nax.max. voltage across the input tuning condenser for optimum setting of both tuning condensers with the tube lighted) against the value of the balance capacity in micro-micro-farads, all at a 200 meters wavelength, which is in general the most critical portion of the range of an amplifier of this. type designed for broadcast reception It will be observed that for R0=0 (i. e. when the present invention is not employed) the primary regenerative rise, i. epEo/Eo, due to the unbalanced primary current, 2', in the coil L is nowhere less than 2. That is, even when the circuit is completely balanced for reactance by ad justment of C the primary regenerative rise is sufficient-to double the gain at short wavelengths, Moreover, the balance tolerance is seen to be very narrow, and it is found in .practice that such. a narrow tolerance does not, in commercial work, accommodate the extreme variations in tube capacities. For example, although the average grid-plate cathe present invention) it will be seen that over a considerable range of balance capacity the ratio of Eo/Eo is substantially equal to unity, which means that there is substantially no primary regenerative rise.

This condition is highly desirable, since it 1 produces a substantially truly one-way amplifier i. e. one which is perfectly balanced over the wavelength range for which it is intended, and in which there is substantially no regeneration or retroaction (the amplifier being of course properly by-passed and shielded). It will be observed that the ratio is substantially equal to unity over a sutficient range of balance capacity to accommodate the commercial variation in tube gridplate capacity, so that when. the'circuit is once balanced for any particular tube, it remains balanced for any substituted tube, and not only does not oscillate, but is: substantially a non-regenerative one-way amplifier circuit. Moreover, by still further increasing R and varying the other constants of the circuit, the amplifier can be made to degenerate, especially at short wavelengths unity), which is found to be desirable in certain cases.

Figure 5 shows gaincurves for circuits having the constants given above in connection with Figure 1, buthaving R =0 and 11 7 00 ohms, respectively. The curves are obtained by plotting the single-stage gain E/Eo, against wavelength in meters, over the wavelength range of 20055() meters. It will be observed that substantially the same desirable and advantageous shape and slope of gain curve obtained by the use of the invention described and claimed in the application of Frederick H. Drake, Ser. No. 275,- 17 8, filed May 4, 1928, may be retained when the present invention is employed, while the added advantages described in connection with Figures 3 and 4 are obtained.

The invention is not limited to the use of R of such a value as to produce a complete resistance balance of the network, but extends to all cases in which the resistance of the balance circuit, including the balance capacity, C is increased beyond the normal resistance (due to di-electric loss, and other resistance) of the capacitive impedance (whether a physical condenser or so-called natural capacity) in the balance circuit;

although this increased resistance of the bala (i. e. the ratio Eo/Eo can be made less than ance, circuit may be incorporated, if desired, in the capacitive balance impedance by making its self-resistance greater than nor mal. The other'specific circuit arrangements and circuit constants described herein, are merely examples of the present invention, which includes many variations and modifications thereof, and is limited only by the teachings of the foregoing specification, by the prior art, and by the following claims:

Claims:

1. An electrical amplifier stage comprising, in combination, a vacuum tube including at'least an anode, a cathode, and a control grid; an input circuit for said stage connected to said control grid; an output circuit including an output coupling coil and resistance in excess of the normal resistance of said coupling coil connected between said anode and said cathode; a tuned circuit including in series a second coil coupled to said output coil, a tuning condenser, and a third coil having negligible electromagnetic coupling with said output coil; and a balancing condenser and resistance connected between said control gridand the junction of said second; and third coils,

2. electrical amplifier stage including vacuum tube having at least'ananode, a cathode, and a control grid; an input circuit associated with said control grid; a tuned output circuit including in series a tuning condenser having one terminal connected to said cathode, and two, coils having negligible electromagnetic coupling with each other, one of said coils being coupled to a coil in a circuit including resistance in excess of the normal resistance of said coupling coil and connected between the said anode and cathode; and a capacitive and resistive impedance connected between said control grid and one of said coils at such a point that said stage may be balanced to reduce retroactive effects due to coupling between said output and input circuits.

3. A multi-stage electrical amplifier circuit comprising, in combination, a plurality of electron tubes each including at least an anode, a cathode, and a control electrode; an input circuit associated with the cathode and control electrode of the first of said electron tubes; a coil and resistance in excess of the normal resistance of said coil associated with the anode and cathode of said first tube; a second coil electromagnctically coupled to said first coil in the reverse sense and having one terminal connected to the control electrode to the second of said electron tubes; a third coil having one terminal connected to a second terminal of said second coil and a second terminal connected to the cathode of .said second tube, said third coil having negancing circuit being connected between said control electrode of said first tube and the junction of said second and third coils; a tuning condenser connected between said first terminal of said second coil and said second terminal of said third coil and across the control electrode and cathode terminals of 'said second tube; and a connection to ground common to said cathodes of said first and sec ond'tubes, and to the junction of said tuning condenser and said third coil.

4. A balanced electrical amplifier circuit including a tube having at least an anode, a cathode, and a control grid; capacitive coupling between said anode and control grid; a coil connected in the anode circuit of said tube; a second coil coupled to said first coil; a third coil constituting with said second coil serially connected elements of a resonant output circuit; a balancing capacitive impedance connected between said control grid and a oint in said output circuit, the ratio of said baancing capacitance to said coupling capacitance between anode and control grid being substantially equal to the ratio, of the mutual inductance between said second coil and said first coil in the anode circuit, to the self-inductance of said third coil, to produce a substantial reactance balance of the electrical network for currents in said resonant output circuit; resistance added to said anode circuit in excess of the normal self-resistance thereof and of an order effective to substantially reduce the amplification at the shorter wave lengths; and means for producing a substantial resistance balance of said electrical network in the presence of said substantial reactance balance thereof and in the presence of said added resistance in the anode circuit.

5. An electrical amplifier circuit comprising, in combination, a vacuum tube including at least an anode, a cathode, and a control grid; an input circuit associated with said control grid; an anode circuit connected between said anode and cathode and including an output coil and resistive impedance in excess of the normal self-resistance of said coil; a second coil coupled to said output coil; a third coil connected in series with said secand coil but having substantially negligible electromagnetic coupling therewith; and. a balance circuit connected between said control grid and the-junction of said second and third coils, and including a balancecapac-ity and resistive impedance in excess. of the normal resistance of said balance capacity.

6. An electrical amplifier circuit comprising, in combination, an electron tube including at least ananode, a cathode, and a control means; an input circuit associated with said control means; a circuit associated with said anode and cathode and including a coil; a second. coil coupled to said first coil and tin ranged so as tohave effectively negative mutual inductance therewith; a third coil connected to said second coil but having substantially negligible electromagnetic coupling therewith; a connection including capacitive impedance between said control means and the junction of said second and third coils; and a connection between said cathode and said third coil; the resistive impedance of said circuit associated with said anode and cathode of said tube being of such value as to reduce the short wavelength amplification of ing, in combination, a vacuum tube includ ing. at least an anode, a cathode, and a con, trol grid; an input c1rcu1t for sand stage con-.

nected to said control grid; an output circuit including an output coupling coil, and resistance in excess of the normal resistance of said coupling coil connected between said anode and said cathode; a tuned circuit including in series a second coil coupled to said out- 7 put coil, atuning condenser, and a third coil having. negligible, electromagnetic coupling with said output coil; and a balancing condenser and resistance connected in series between said control grid and the junction of said second and third coils.

8. Anelectrical amplifier stage comprising, in combination, a vacuum tube including at least an anode, a cathode, and a control grid; an input circuit for said stage connected to said control grid; an output circuit including an output coupling coil, and resistance in excess of the normal resistance of said coupling coil connected between said anode and said cathode; a tuned circuit including in series a second coil coupled to said output coil, a tuning condenser, and a third coil having negligible electromagnetic coupling with said output coil; and a balancing condenser and shunt resistance connected between said control grid and the junction of said second and third coils. v I I 9. Anelectrical amplifier stage comprising, in combination, a vacuum tube including an anode, a cathode, a control grid, and a positive auxiliary grid, an input circuit. for saidstage connected to said control grid; an

for varying the potential of said positive grid to control the gain of the stage. a

10. A multi-stage electrical amplifier circuit comprising, in combination, a plurality of electron tubes each including at least an anode, a cathode and a control electrode; an input circuit associated with the cathode and control electrode of the first of said electron tubes; a coil and resistance inexcess of the normal resistance of said coil associated with the anode and cathode of said first tube; a second coil electromagnetically coupled to said first coil in the reverse sense and having one terminal connected to the control electrode to the second of said electron tubes; a third coil having oneterminal connected to a second terminal of said second coil and a second terminal connected to the cathode of said second tube,said third coil having negligible electromagnetic coupling with said second coil; a balancing circuit including capacitive and resistive impedance, said balancing circuit being connected between said control electrode of said first tube and the junction of said second and third coils; a tuning condenser connected between said first terminal or" said second coil and said second terminal of said third coil and across the control electrode and cathode terminals of said second tube. 1 v V In testimony whereof, we afiix our signatures.

' FREDERICK H. DRAKE.

PAUL FAR NHAM.

output circuit including an output coupling coil, and resistance in excess of the normal resistance of said coupling coil connected be- ;ed between said control grid and the junction of said second and third coils; and means 

